1. Field of the Invention
The present invention concerns a method for the rear illumination of a liquid crystal matrix display panel, as well as the fluorescent tubes illumination device implemented according to the invention.
Liquid crystal display (LCD) screens or panels are light modulators wherein the transmission of each of the symbols to be represented is controlled electrically. The matrix panels are formed by dots (or pixels) which are distributed evenly in lines and columns throughout the surface and are controlled electrically by a matrix network of lines and columns. The image is refreshed periodically at a frame rate, the scanning being done line by line at a line rate. For a matrix panel with a total of N lines, the electrical information giving excitation is conveyed to the successive pixels of one and the same line for a period T/N, T being the total duration of scanning of the panel or frame duration. During the remaining interval of time, i.e. T(l-1/N) of each frame, the pixels are not excited. This periodic variation in excitation voltage leads to a ripple in the optical transmission which causes a visual flicker effect, which is all the more visible as the refreshing time is small. This phenomenon also increases with temperature.
When the maximum transmission that can be achieved, especially for polychrome panels is low, it is preferable to use an illumination device in the rear of the panel rather than a scattering/reflecting device which reflects the ambient illumination.
This rear illumination device is usually made by means of fluorescent tubes. To obtain satisfactorily uniform and high luminance on the entire rear face of the panel, it is necessary to use an assembly of several tubes, generally placed in parallel. The uniformity or homogeneity can be improved by using a scattering device placed between the tubes and the panel, as well as a reflector placed behind the tubes.
To modify the illumination, it is possible to control the luminance of the fluoresent tubes.
A prior art technique consists in using a DC/AC converter or inverter which takes a low DC voltage and gives the tubes a high-frequency AC voltage of some hundreds of volts designed to light the fluorescent tubes at each alternation. According to this technique, the variation in mean luminance of the tubes is obtained by all or nothing modulation, periodically at a low frequency rate, of the high-frequency AC voltage applied to the tubes. To this effect, a pulse with modulation of adjustable duration is produced: it can be used to cause variation in the mean intensity of the electrical current in the tubes and, consequently, in the luminance of the tubes. This pulse is repeated at the low frequency rate. If the low frequency of modulation is chosen to be sufficiently high (hundred or several hundreds of Hertz), the temporal luminance ripple that results therefrom is fast enough to be invisible.
This prior art technique does not, however, remove the flicker effect due to the refreshing frequency which is quite low and may have a period of 10 to 20 milliseconds for example.